Alloy steels



Patented Apr. 18, 1939 UNITED STATES PATENT OFFICE United States Steel Corporation, New York,

N. Y., a corporation of New Jersey No Drawing. Application October 22, 1936, Serial No. 107,104

2 Claims.

The present invention relates toalloy steels.

In the strict sense, steel may be said to be an alloy of iron and carbon. However, in a metallurgical sense, in which the term is used in this specification, an alloy steel is steel (that is, iron and carbon) alloyed with another chemical element or other chemical elements.

An object of the present invention is to produce an alloy steel which is well adapted to re; sist failure by fatigue, especially for use in highly stressed parts, such as springs.

A further object is to produce an alloy steel for general purposes which involves no special equipment in its manufacture and which has improved characteristics over steel as heretofore known and used.

A further object is to provide a steel well adapted for use in cushioning springs, which steel will effectually resist deterioration in service.

A further object is to provide an alloy steel in which the characteristics of titanium are utilized to advantage in the manufacture of alloy steels.

A further object is to provide an improved steel well adapted to meet the needs of commercial operation.

Further objects will appear as the description proceeds.

The present invention contemplates the accomplishment of the objects above referred to by providing a shallow hardening alloy steel. To the skilled steel manufacturer the term shallow hardening alloy steel has a definite significance, meaning a steel which, when heated to above its critical temperature and then cooled, has a relatively pronounced tendency to change rapidly from the austenitic to the pearlitic type of internal structure. Steel of fine grain structure tends to be shallow hardening, though of course certain alloy steels, even though they be fine grained, may not be shallow hardening in comparison with the steels for which invention is claimed herein. Of course it is possible to produce shallow hardening steel by using a very restricted amount of carbon, and such steel has its uses in the arts. The present invention is limited to steels in which the carbon content is equal to or greater than .45% by weight.

In defining the degree of shallow hardening contemplated in the present invention it may be observed that certain alloy steels, as for example nickel chromium steel, have characteristics differing from steels contemplated in .the

present invention. As is well known, alloy steels as ordinarily known to metallurgists will, when heated to a temperature of say 1600 deg. F., form austenite, which is understood by metallurgists to be-a solution of carbon in iron. As the 5 steel is cooled from the temperature referred to, the structure changes, and when the steel reaches the neighborhood of about 900 deg. F. there is a tendency to form pearlite, which is a tough substance of low hardness. In the case of most alloy steels as now known and used, as for example the nickel chromium steel above referred to, the length of time in the temperature zone in the neighborhood of 900 deg. F. required for the formation of pearlite is in the. neighborhood of seconds. The temperature zone referred to may range from 1050 to 850 deg. F. In the case of the alloy steels forming the subject matter of the present invention, the period of time required in the 900 deg. zone 90 referred to is in the neighborhood of 8 seconds. Considering a body of steel which has been heated to say about 1600 deg. F. and quenched in water, it willbe readily understood that in the case of nickel chromium steel or the like the period of dwell in the 900 deg. zone referred to may be too short to form pearlite, but said period of dwell may be ample for the formation of pearlite in the composition of shallow hardening steels. In other words, using the example above referred to, the period of time in which the center of the member under test is in the temperature zone of 900 deg. F. above referred to may be-less than 20 seconds but more than 8 seconds. formed in the quenching operation, the alloy steel, upon further cooling, will form martensite, which is a hard material. ,In the event that pearlite has formed in the center of the member under test in the quenching operation, this pearlite will retain its identity when the member under test has cooled to atmospheric temperature. The above example illustrates a measure which may be applied to define the degree of hardenability contemplated in the present invention. For convenience, the measure of hardenability may be expressed in connection with an alloy steel bar. The present invention contemplates a composition of alloy steel such that when a cylindrical bar of 1 /2 inches or more in diameter of homogeneous composition is heated to a temperature of approximately 1600 deg. F. and then quenched in still water at room temperature, the material at the axis of said bar will, upon cooling to room temperature, have a In the event that pearlite has not 35 Rockwell hardness of not more than 50-C. Expressed in another way, and referring to the quenching of the alloy steel in oil, the present invention contemplates a composition alloy steel such that when a cylindrical bar of 1 inch or more in diameter of homogeneous composition is heated to a temperature of approximately 1600 deg. F. and then quenched in still oil at room temperature, the material at the axis of said bar will have a Rockwell hardness of not more than 50-C.

It will be understood, of course, that the references immediately above made to bars of specified diameters are merely for convenience of definition of one property of these steels. The steels, of course, are useful in members of all dimensions including small sections in which the steels harden through to their axes or midsections.

Titanium has the advantage that it produces fine grain structure and is relatively low in price. As is well known, the cheapest form of titanium 0n the market at the present time is that which has been reduced by treatment with carbon resulting in what is known as a high carbon ferro titanium. In some arts, the high carbon content is objectionable but in the manufacture of alloy spring steel in which the carbon content is rel.. atively high, the high carbon content of the low priced high carbon ferro titanium is not objectionable.

Titanium has the advantage that it tends to produce fine grain structure and shallow hardening over a relatively wide temperature range in the hardening operation. Moreover, when using titanium in the manufacture of alloy spring steel the amount of aluminum may be materially reduced from present practice in the melting operation to secure fine grain structure and shallow hardening.

In carrying out the present invention it is contemplated to use in the manufacture of the improved alloy steela predominating amount of titanium either with or without other shallow hardening elements, such for example as vanadium or equivalent. Correspondingly, the present invention contemplates a-relatively low amount of the deep hardening elements such for example as manganese, chromium and/or nickel). The relative terms referred to in the preceding sentence have reference to the functions of the alloying elements referred to as commonly used in alloy steels in common use today.

Proceeding to a more specific definition of alloying elements contemplated in the present invention, the following observations may' be made:

Since titanium reaches its full effectiveness for grain refinement at approximately 0.06% by weight, a predominating amount of titanium in the alloy steel according .to the present invention would be approximately 0.02 to 0.06% by weight.

Since vanadium reaches its full eifectiveness at about 0.15% to 0.20%, a predominating amount of vanadium would be about 0.075 to 0.20% by weight.

Since nickel reaches its full efiectiveness at about 3% to 4%, a predominating amount of nickel would be about 1%-4% by weight.

Therefore, expressing the purpose of the present invention in other language, it may be said that the present invention contemplates the use of titanium in the amount of- 0.02 to 0.06% by weight, said element beingiused without chromium or manganese if preferred; or, if chromium be used, then with less than 375% by weight of chromium; or, if manganese be used, then with less than 1.25% of manganese. -Expressed. in

still other language, certain advantages of the present invention will be had if the shallow hardening elements (for example titanium, with or without vanadium or equivalent) are used to a percentage equal to at least 50% of their fully effective amount in opposing hardening in quenched steels; and certain advantages of the present invention will be had if the deep hardening elements (for example chromium, manga-' nese and/or nickel) are restricted to not more than 50% of their fully effective amounts as hardening agents in quenched steels. Another way of defining the limits of the materials contemplated in the present invention is by reference to the critical cooling rate of the materials. Critical cooling rate of steels may be defined as that rate above which a steel will harden in the cooling process and below which a steel will not.harden. When the term "harden is used in this connection it is intended to convey the meaning that the material of the steel becomes the hard material martensite. The

present invention contemplates -steels which when quenched in a quenching medium at atmospheric temperature will just harden if it passes 1325 deg. F. at a rate above 50 deg. per

may be recited:

Per cent Carbon from .85 to .95 Manganese from .20 to .50 Silicon from .10 to .35 I Chromium from .20 to .50 Titanium from .03 to .06

Balance largely iron with traces of impurities. It will be understood, of course, that the analysis just noted is illustrative only, and the objects of the present invention may be realized by proper balancing of shallow hardening elements against deep hardening elements. For example, the carbon may vary from .30% to 1%; the manganese may vary from .40% to 1.25%; the chr0- mium may vary from 0 to 35%; the titanium may vary from 0.01% to'0.10%. A choice of the elements referred to, to provide a shallow hardening steel meeting the tests recited in this specification, will fall within the scope of this invention.

Anyone familiar with the manufacture of alloy steels has readily available to him information as to the constituents and the amounts thereof which must be used in producing the steels having the foregoing analyses.

In arriving at the object of making a shallow hardening alloy steel, the present invention contemplates a steel of fine grain size. In referring to fine grain size it is to be understood that reference is had to the grain size of austenite, which is the structure composing the steel when the steel has been heated to a high temperature (for example 1600 deg. F.) preparatory to hardening by quenching. The term fine grain" or fine grain size asused in this specification means from #5 to #8 on the ASTM chart of grain sizes.

. This may be further explained as meaning more than 12 grains per square inch when examined at a magnification of 100 diameters.

The fact is well known that fine grain size tends to shallow hardening. It is also well known that very fine grains, particularly #7 and #8, according to the ASTM chart, exhibit structural features known as abnormality, meaning thereby a particular distribution of the iron carbide (that is, cementite) observed in the carburizing test (McQuaid-Ehn carburizing test), which testis sometimes used in determining grain size.

In order to arrive at fine grain size (including abnormality) it has been common to add, in the furnace, in the ladle or in the molds, about one pound of aluminum per ton of steel. It has been discovered that if titanium be used as one of the shallow hardening elements, the amount of aluminum may be materially reduced. Titanium tends to produce fine grain structure and shallow hardening in steel over a relatively wide tempera- "ture range in the hardening operation.

As is well known, silicon is used in the manufacture of alloy quality steels for the purpose of combining with the oxygen and thereby removing said oxygen. An excess of silicon always ap- Accordingly, inthe appended claims the term traces of impurities will include this excess silicon.

The alloy steels forming the subject matter of this invention are characterized by a high degree of ductility, toughness, resiliency and resistance to shock and fatigue.-

Though certain preferred analyses and steps have been recited in the foregoing specification, many modifications will occur to those skilled in the art. It is intended to cover all such modifications that fall within the scope of the appended claims.

What is claimed is:-

l. A shallow hardening alloy steel which is composed of the following ingredients substantially in the amounts specified:

Per cent Carbon from .85 to .95 Manganese from .20 to .50 Chromium from .20 to .50 Titanium from .03 to .06

Balance iron with traces of impurities.

2. An alloy steel characterized by a high degree of resilience and resistance to shock and fatigue which is composed of the following in gredients within the ranges specified and balanced against each other to produce a shallow hardening steel:

Per cent Carbon from .45 to 1 Manganese from .40 to 1.25 Chromium trace to .75 Titanium from .01 to .10

Remainder iron with traces of impurities.

MARCUS A. GROSSMANN. 

